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Earth’s middle shrinks and grows

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Really. Earth may look perfectly spherical from space, like a giant marble, but it actually isn't! Instead, our planet is slightly wider around the equator because matter is forced outward by centripetal force as the Earth spins. That's the force you feel on the carnival "Tilt-a-Whirl", where you would go flying outward if you weren't restrained.

However, this bulge at the Earth's midsection isn't constant; it shrinks and grows. Standing on Earth's surface, we can’t see the Earth's shape, much less changes in it. But using satellites researchers watched the equator grow smaller over the past 20 years or so. They believed this had been happening at least since the last ice age 18,000 years ago. Since then, as temperatures warmed and glaciers at the poles melted slowly, little by little the poles became less squashed under heavy ice. Responding to this change, molten rock moved under the Earth's crust from the equator to the poles to fill in the new space and the equator grew smaller.

More recently, however, researchers have found that the equator is again growing. Why? We haven't started a new Ice Age, squashing the poles again, so what is forcing material back to the equator? One hypothesis is that a small change in Earth's magnetic field might be responsible. Another idea is that, if glaciers are melting, they could be adding more water to the equator via ocean currents. This increased mass could bulge the equator a bit.

But these are only guesses. Whatever the reason, researches believe that the change in Earth's shape is natural and has nothing to do with human activity.

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